Dense red spots on the arms are usually caused by excessive squeezing, allergic purpura, perikeratosis and other factors. 1. Excessive squeezing: when the arm is squeezed too much by external forces, it can make the local capillaries rupture, resulting in dense red spots on the arm. 2. allergic purpura: a vascular allergic reaction disease, because the body is allergic to certain substances (such as bacteria, viruses, food, drugs, etc.), resulting in immune dysfunction, so that the permeability of the capillary wall and the fragility of the capillary wall increases, resulting in patients with limbs and other parts of the dense purple-red spots, and may be accompanied by abdominal pain, arthralgia, and other manifestations. 3. Periosteal keratosis: the cause of the disease is not yet clear, and genetic, vitamin A deficiency and other factors related to the patient’s outer upper arm and other parts of the dense reddish pimples, chicken skin-like appearance, and the skin has a sense of roughness. In addition, there are many other reasons that can cause dense red dots on the arm, it is recommended that patients go to the hospital in time to exclude non-disease factors, and receive treatment as soon as possible.